The invention relates to an apparatus for examining objects by ultrasonic echography, including an array of m ultrasonic transducer elements associated with a stage for the transmission of ultrasonic signals to the object to be examined and with a stage for receiving and processing echographic signals returned to the transducer elements by the object being examined, the transmission stage and receiving and processing stage respectively including means for focusing by the provision of suitable delays in n transmission channels in n receiving channels associated with n transducer elements, among said m elements, forming an ultrasonic aperture, the receiving and processing stage further including.
associated with the n receiving channels, means for determining (n-1) correction values for the focusing delays; and PA1 means for the correction of the focusing delays as a function of the (n-1) values thus determined. PA1 a stage for generating a time window signal defining a time window during which the i.sup.th corresponding correction value is determined; PA1 a stage for determining, the phase shift between the focused echographic signals of the ranks (i) and (i+1) received by said correction circuit, said phase shift constituting the i.sup.th corresponding correction value to be determined during the time period or being directly related to it.
The operating principle of current echographs is generally based on the assumption that the velocity of ultrasonic waves in the tissues being examined is constant, particularly to enable the different delays due to beam focusing and inclination to be computed and to enable the information relating to the transit times of the echoes to be converted into depth information. However, this assumption is seldom true: the propagation velocity of ultrasonic waves has, for example, an average value of 1540 m/s in the liver and is approximately 1300 m/s in fatty tissues. This results essentially in a defocusing of the ultrasonic beams, both in the transmission and the receiving mode, leading to loss of resolution and contrast of the images, which loss increases as larger focusing apertures and waves of higher frequency are used. The influence of the frequency on the resulting degradation can be understood when bearing in mind that an accuracy of approximately one eighth of a wavelength is to be observed for the delays, i.e. an accuracy which improves as the frequency increases, while the influence of the aperture size can be explained by bearing in mind that the probability of zones of different velocities of sound being encountered increases as the aperture is larger.
A solution adopted to reduce the disturbances as a result of such a defocussing is to compare the received echographic signals by correlation. U.S. Pat. No. 4,471,785 describes an apparatus in which, in order to cope with differences in the velocity of ultrasonic waves in the objects being examined and the resulting image artefacts, a correlator arranged in parallel with n reception channels enables (n-1) focusing delay correction values to be determined, which values are applied to delay lines in these reception channels and which, prior to combination of the echographic signals, modify in a specific way the delays introduced by said lines in each channel in view of receiving-mode focusing.